The present invention relates to a process for producing mineral oil from underground formations, wherein, in one process step, permeable regions of the underground formation are blocked by injecting aqueous formulations of hydrophobically associating copolymers into the formation.
In natural mineral oil deposits, mineral oil occurs in the cavities of porous reservoir rocks which are closed off from the surface of the earth by impervious covering layers. The cavities may be very fine cavities, capillaries, pores or the like. Fine pore necks may, for example, have a diameter of only approx. 1 μm. The underground formation may additionally also have regions with pores of greater diameter and/or natural fractures. In addition to mineral oil, including proportions of natural gas, a deposit generally comprises water with a greater or lesser salt content.
After a well has been sunk into the oil-bearing strata, mineral oil at first flows to the production wells owing to the natural deposit pressure, and is flushed to the surface of the earth. This phase of mineral oil production is known by the person skilled in the art as primary production. However, flush production generally ceases very rapidly, especially under poor deposit conditions, for example a high oil viscosity, rapidly declining deposit pressure or high flow resistances in the oil-bearing strata. With primary production, it is possible to produce an average of only 2 to 10% of the oil originally present in the deposit. In the case of higher-viscosity mineral oils, flush production is generally completely impossible.
In order to enhance the mineral oil yield, what are known as secondary and optionally tertiary production processes are therefore used.
One secondary mineral oil production process is called “water flooding”. For this purpose, the deposit is provided with one or more injection wells in addition to the production wells, i.e. the wells through which mineral oil is withdrawn from the mineral oil formation. Water is injected into the oil-bearing strata through the injection wells. This artificially increases the deposit pressure and forces the oil from the injection wells in the direction of the production wells. Water flooding can significantly enhance the exploitation level. Instead of water, it is also possible to inject steam into the deposit (“steam flooding”). This is advisable especially when the deposit comprises high-viscosity oils.
In the course of water flooding, in the ideal case, a water front proceeding from the injection well should force the oil homogeneously over the entire mineral oil formation to the production well. In practice, a mineral oil formation, however, has regions with different levels of flow resistance. In addition to fine-porosity, oil-saturated reservoir rocks with a high flow resistance to water, there also exist regions with a low flow resistance to water, for example natural or synthetic fractures or very permeable regions in the reservoir rock. The permeable regions may also be already exploited regions. In the course of water flooding, the flooding water injected naturally flows principally through flow paths with low flow resistance from the injection well to the production well, while there is at least slower flow, if any, of water through the fine-porosity, oil-saturated deposit regions with high flow resistance. The water thus no longer flows homogeneously through the formation, and the water front is instead very irregular (called “fingering”), and an increasing amount of water and a decreasing amount of mineral oil are produced via the production well. In this connection, the person skilled in the art refers to “watering out of production”. The effects mentioned are particularly marked in the case of heavy and viscous mineral oils. The higher the mineral oil viscosity, the more probable is rapid watering out of production. The problem occurs especially in the presence of fissured rock formations (called “fractured reservoirs”).
There also exist mineral oil formations in which a water-bearing stratum is arranged below an oil-bearing stratum. In the course of drilling into such a formation, not only mineral oil but also water is produced, and so production here too is significantly watered out.
There has been no lack of attempts to prevent the inhomogeneous flow of water, or at least to achieve more homogeneous flow. In the prior art, there are therefore known measures for closing such highly permeable zones between the injection wells and production wells by means of suitable measures, or at least for reducing the permeability thereof. As a result, the flooding water or flooding steam is forced again to flow through the oil-saturated, low-permeability strata, and further mineral oil can thus again be mobilized. Such measures are also known as “conformance control” or “water shut-off”. An overview of conformance control measures is given by Boiling et al. “Pushing out the oil with Conformance Control” in Oilfield Review (1994), pages 44 ff.
For blocking of highly permeable regions of underground formations, i.e. for conformance control, it is possible to use comparatively low-viscosity formulations of particular chemical substances which can be injected readily into the formation, and the viscosity of which rises significantly only after injection into the formation, under the conditions which exist in the formation. Such formulations comprise inorganic, organic or polymeric components suitable for increasing viscosity. The rise in viscosity of the injected formulation can occur, for example, with a simple time delay, and/or the rise in viscosity can be triggered by the temperature rise when the injected formulation in the deposit gradually heats up to the deposit temperature. Formulations whose viscosity rises only under formation conditions are known, for example, as “thermogels” or “delayed gelling systems”.
SU 1 654 554 A1 discloses a process for producing oil using mixtures of aluminum chloride or aluminum nitrate, urea and water, which are injected into the mineral oil formation. The formulations naturally flow preferably along the flow paths with the lowest flow resistance. At the elevated temperatures in the formation, the urea is hydrolyzed to carbon dioxide and ammonia. The release of the ammonia base significantly increases the pH of the water, and a high-viscosity gel of aluminum hydroxide precipitates out, which blocks the highly permeable zones.
US 2008/0035344 A1 discloses a mixture for blocking of underground formations with retarded gelation, which comprises at least one acid-soluble, crosslinkable polymer, for example partly hydrolyzed polyacrylamide, a partly neutralized aluminum salt, for example an aluminum hydroxide chloride, and an activator which can release bases under formation conditions, for example urea, substituted ureas or hexamethylenetetramine. The mixture can preferably be used at a temperature of 0 to 40° C. and gelates at temperatures above 50° C., according to the use conditions, within 2 h to 10 days.
RU 2 361 074 discloses a process for blocking highly permeable zones, in which portions of formulations based on urea and aluminum salts are injected into a deposit with high deposit temperature.
U.S. Pat. No. 4,182,417, US 2007/0204989, WO 20007/126318 A1 and WO 2010/069607 A1 disclose water-swellable particles for blocking of underground formations. These particles can be injected in a suitable formulation into the underground formation, swell in the formation under the influence of the formation water and in this manner block highly permeable regions of the formation.
R. D. Sydansk “Acrylamide-Polymer/Chromium(III)-Carboxylate Gels for Near Wellbore Matrix Treatments” in Proceedings Society of Petroleum Engineers/US Department of Energy, 7th Symposium on Enhanced Oil Recovery, Apr. 22-25, 1990, Tulsa, Okla., SPE/DOE 20214, Society of Petroleum Engineers, 1990 disclose acrylamide-chromium(III) carboxylate gels for blocking of underground formations. For this purpose, acrylamide and Cr(III) carboxylates, for example Cr(III) acetate, are injected into the formation. Under the formation conditions, amide groups of the polymer are hydrolyzed to carboxylate groups. The Cr(III) carboxylate then crosslinks carboxylate groups of different polymer strands, thus forming a viscous gel.
U.S. Pat. No. 4,613,631 discloses gels formed from crosslinked polymers for blocking of underground formations. The polymers may, for example, be polyacrylamide, polyacrylic acids, or else biopolymers, for example xanthogenates. The crosslinkers used are organic compounds which have at least two positively charged nitrogen atoms.
U.S. Pat. No. 7,150,319 B2 discloses a process for blocking underground formations, in which a copolymer comprising 2-acrylamido-2-methylpropanesulfonic acid, acrylamide, a further nitrogen-containing monomer, for example N-vinylformamide or N-vinylpyrrolidone, and vinylphosphonic acid as monomers. The copolymer is crosslinked with compounds of chromium, zirconium, titanium or aluminum to form a viscous gel.
U.S. Pat. No. 6,803,348 B2 discloses a process for reducing water production from mineral oil-bearing underground formations, in which water-soluble, hydrophobically associating copolymers comprising a linear hydrophilic main chain, hydrophobic side groups and functional groups which can be used for crosslinking are used. The water-soluble copolymers are injected into the underground formation and crosslinked therein, for example by means of Cr(III) ions, Zr(IV) ions or aldehydes. The polymers are preferably based on polyacrylamides. The hydrophobic groups are preferably alkyl groups having at least 6, preferably at least 8 and more preferably at least 12 carbon atoms. The copolymer may comprise, for example, N-alkylacrylamides, for example N-decylacrylamide, as a monomer.
WO 2010/133527 A2 discloses hydrophobically associating copolymers which comprise at least hydrophilic, monoethylenically unsaturated monomers, for example acrylamide, and monoethylenically unsaturated, hydrophobically associating monomers. The hydrophobically associating monomers have a block structure and have—in this sequence—an ethylenically unsaturated group, optionally a linking group, a first polyoxyalkylene block which comprises at least 50 mol % of ethyleneoxy groups, and a second polyoxyalkylene block which consists of alkyleneoxy groups having at least 4 carbon atoms. The application discloses the use of such copolymers as thickeners, for example for polymer flooding, for construction chemical applications, or for detergent formulations.
WO 2011/015520 A1 discloses a process for preparing hydrophobically associating copolymers by polymerizing water-soluble, monoethylenically unsaturated, surface-active monomers and monoethylenically unsaturated hydrophilic monomers in the presence of surfactants, and the use of such copolymers for polymer flooding.
Our prior applications EP 10192323.3, EP 10192334.0 and EP 10192316.7 disclose the use of the hydrophobically associating copolymers disclosed in WO 2010/133527 A2 in specific processes for polymer flooding.
However, none of the latter cited applications discloses the use of such hydrophobically associating copolymers for blocking of underground mineral oil-bearing formations.